This invention relates to the art of preparing a graft copolymer by grafting from a polymer backbone (hereafter "backbone"), so that an oxirane polymer grafted from the backbone has a clearly defined chain length and a narrowly defined, controlled molecular weight (mol wt), substantially free from homopolymer residues. The graft copolymers of this invention are not formed by grafting onto a backbone, therefore the process is characterized by very high, if not nearly perfect grafting efficiency. (see "Cationic Grafting: A Critical Review" by Kennedy, J. P., J. Appl. Poly. Sci.: Appl. Poly. Symposium 30, 1-11, 1977). By "graft copolymer" I specifically refer to a copolymer of one polymer attached to the backbone of another polymer which is linear, at various points along the backbone. Nothing in the prior art enables one to prepare such a graft copolymer by cationic ring-opening polymerization. With respect even to the general art of cationic polymerization, Prof. Joseph Kennedy said "Cationic polymerizations have only very recently emerged from the dark middle ages and reached the point where systematic tailoring of polymer structures became possible." (see Isotopics, pg 4, May 1982)
In the known art of synthesis of graft copolymers by ring-opening polymerization, whether anionic or cationic, there is no suggestion how a graft copolymer with tailored chain lengths of grafted polymer chains may be made. Thus there is no basis for assessing the properties of such a graft copolymer. Further, it will be appreciated that an assessment of the probability that the little-understood process of cationic ring-opening polymerizaton of a monomer to a backbone with pendant hydroxyl (OH) groups will provide a graft copolymer with any particular structure and desirable properties such as controllable block length and thermal stability, is speculative.
The difficulty of tailoring related block or graft copolymers, generally, by cationic ring-opening polymerization so as to provide a mol wt within a narrow range and well-defined functionality, or with high graft efficiency free from homopolymer, is well known. (see Kennedy publications, supra). Recently, novel block and graft copolymers based on the di- and multicationically ended living polyacetals and polyethers have been shown to be formed without a hydroxyl-containing material, and under anhydrous conditions. See Cationic Ring-Opening Polymerization, by Stanislaw Penczek, Polish Academy of Sciences, Center of Molecular and Macromolecular Studies, 90-362 Lodz, Poland; see Makromol. Chem., Suppl., 3, 17-39 (1979). This work appears to confirm the teachings of the Dreyfuss U.S. Pat. No. 3,850,856 that OH groups would `kill` the system, if it were a living one.
This invention is particularly directed to a process for making certain graft copolymers by polymerizing an oxirane monomer from a hydroxyl-containing substantially linear backbone polymer (hereinafter "HCB", for brevity) which serves as the chain propagator furnishing pendant OH groups as chain propagation sites from which the oxirane polymer is grafted; and, which process, relies upon a living polymerization generated by the catalytic action of a fluorinated acid catalyst having the formula HMF.sub.6 wherein M is selected from phosphorus, arsenic and antimony; or, an oxonium salt of the acid. The process of my invention yields graft copolymers which persons skilled in the art were unable to make with the teachings of the prior art, or believed they could not make, before the discovery of my process.
More specifically, the graft copolymers of my invention are made by a living cationic ring-opening polymerization of an oxirane monomer grafted from sites furnished by the HCB in the presence of a known catalyst disclosed in U.S. Pat. No. 3,585,227, and in U.S. Pat. No. 3,850,856, the disclosures of which are incorporated by reference herein as if fully set forth. The polymerization disclosed in U.S. Pat. No. 3,850,856 was based on water, or ethylene glycol terminating the growing polymer chains by introducing hydroxyl (OH) groups at the terminal positions. This polymerization has now been discovered to be a living polymerization as described more fully in copending patent applications Ser. Nos. 418,341 filed Sept. 9, 1982 now U.S. Pat. No. 4,485,211 and 427,370 filed Sept. 29, 1982, now U.S. Pat. No. 4,451,618, the disclosures of which are incorporated by reference thereto as if fully set forth herein. This living polymerization is quite surprising since it is well known that "The nature of the processes involved in cationic copolymerisations of monomer mixtures, and the experimental limitations which they impose, have made this synthetic route to block copolymers generally unsatisfactory, at least for most comonomer pairs." (see Block Copolymers, by Allport, D. C. and Janes, W. H., pg 354, John Wiley & Sons, 1973). Implicitly, this corroborates a prejudice against cationic living polymerization systems.
Further, since known polymers such as those described in the '856 patent are clearly stated to be hydroxyl-ended, because of the use of water or glycol, there was no motivation to explore the possibility that a backbone wih plural pendant OH groups intermediate its ends might function as polymer grafting sites because the manner analogous to that in which water or a glycol furnish chain propagating sites was unknown. Nor was it then realized that the choice of propagator (having OH propagating groups), and particularly a propagator having at least five aforementioned pendant OH groups would result in graft copolymers having quite different structures from those of the prior art copolymers.
Nevertheless, it has now been found that, under certain conditions, growth of a graft copolymer is propagated from OH groups in a HCB, which growth occurs in a well-definable living polymerization system. More particularly, it now appears that the structure of the residue of the HCB chain propagator, whatever its length, does not adversely affect the growth of a graft copolymer, but generates a living system able to grow preselected blocks, so that the overall polymer weight, the polymer segmental weight, and the polymer's functionality are each narrowly defined.
Epihalohydrin polymers formed by cationic polymerization with triethyloxonium hexafluorophosphate (TEOP) are known to be formed as disclosed in U.S. Pat. No. 3,850,857. However, neither the molecular weight of the polymers nor their functionality can be narrowly controlled, resulting in the loss of control of the polymers' properties. In contrast, the unexpectedly close control of molecular weight and high grafting efficiency without oxirane homopolymerization obtained with my invention facilitates the production of a variety of tailored graft copolymers which are not only useful as film-formers, viscosity increasing agents, dispersing agents for polymerization, and the like, but also for mineral beneficiation where highly specific properties are essential to the making of sharp, and hence profitable, separations.
Such close control of mol wt and high grafting efficency is only possible because this oxirane cationic ring-opening polymerization has the unique characteristics of a living polymerization in which the mol wt of the polymer is increased directly with the amount of monomer converted to polymer, and each OH group provides a grafting site (propagating site).
Some two decades ago, it was known that hydroxyl group-containing compounds are condensed with epoxyalkyl halides in the presence of fluoboric catalysts as disclosed in U.S. Pat. No. 3,129,232. Soon thereafter it was shown in U.S. Pat. No. 3,305,565 that water is an initiator in the presence of various acid catalysts forming halo-hydroxyl terminated polymers which could be epoxidized. More recently, terminally unsaturated liquid epihalohydrin polymers have been disclosed in U.S. Pat. No. 4,256,910 which are relatively low in molecular weight, in which a homopolymer of an epihalohydrin, or a random copolymer of two or more epihalohydrins, or copolymers of an epihalohydrin with a vicinal epoxide, are formed using the same TEOP catalyst as used in U.S. Pat. No. 3,850,856 process. However, there is neither any indication that the hydroxyalkyl (meth)acrylate used in U.S. Pat. No. 4,256,910 functioned as a chain propagator, nor that the mol wt and functionality of the polymer were narrowly defined.
U.K. Patent Application No. 2,021,606A teaches that hydroxyl-terminated poly(chloroalkylene ethers) have not proven entirely satisfactory when prepared by cationic ring-opening polymerization as disclosed in U.S. Pat. Nos. 3,850,856; 3,910,878; 3,910,879; and, 3,980,579. Thus, the problems inherent in the use of prior art catalysts referred to in the foregoing U.S. patents have been documented. A solution to the problems was provided in the British patent application. This solution was to use a catalyst comprising (i) a fluorinated acid catalyst having the formula H.sub.m XF.sub.n+m wherein X is selected from boron, phosphorus, arsenic and antimony, m is 0 or 1 and n is 3 when X is boron and n is 5 when X is phosphorus, arsenic and antimony, and, (ii) a polyvalent tin compound.
This British patent teaches that only tin fluorometallic compounds even among other Group IV metals, has a peculiar catalytic action not attributable to Group V fluorometallic compounds. With this catalyst, it is suggested that any hydroxyl-containing material ("HCM") having up to six (6) OH groups, whether terminal or pendant, may be used in the formation of a polymer with an alkylene oxide, provided at least about 50% by weight (wt) of the alkylene oxide is a chloroalkylene oxide.
Among such HCMs are disclosed water, and any liquid or solid organic material which has a hydroxyl functionality of up to six (6), whether such material is monomeric or polymeric. Less generally, polyhydric alkanols, haloalkanols and polymeric polyols are disclosed, including hydroxyl-terminated polyalkadienes and polyether polyols.
Specifically disclosed HCMs are glycerol, sorbitol and polyoxyethylene and polyoxypropylene glycols and triols of molecular weights from about 200 to about 2000. My experiments with OH-terminated polyoxyethylene and polyoxypropylene which are detailed in my copending application Ser. No. 427,370, failed to yield block copolymers with epichlorohydrin despite my having continued the polymerization reaction for more than 80 hr., probably due to the back-biting mechanism (see Advances in Polymer Science, Vol 23, pg 128, by E. J. Goethals, published by Springer-Verlag, Berlin Heidelberg New York, 1977) which results in its depolymerization forming a stable 6-membered ring. Only when the equilibrium of the reaction was heavily biased by using a large excess of monomer and catalyst, deliberately, because of the understanding acquired from my present invention, was there a small amount of block copolymer formed.
With glycerol, a branched polymer is also formed. With polyoxypropyelene triol, a three-branched block copolymer is formed. If sorbitol were an effective HCM, a six-branched copolymer might have been expected to be formed. However, sorbitol like cellulose is not soluble in oxirane monomers and no mutual solvent is known which will not deleteriously affect the catalyst. My experiments with sorbitol failed to produce a six-branched copolymer; glucose and sucrose also failed to yield five- and eight-branched chains respectively; and, cellulose failed to yield any copolymer.
Despite the structure written in the British reference, the overly broad disclosure failed to disclose the essential requirements for grafting from pendant OH groups of a polymer backbone so as to form a graft copolymer, and more particularly, failed to recognize that it was essential for the formation of a graft copolymer that the HCB be soluble in the monomer to be grafted from it, even if a mutual solvent is necessary to provide a substantially homogeneous reaction mass.
Amphiphilic block copolymers have been found especially useful in water treatment processes, and as the main constituent of nonaqueous lyotropic liquid crystals. A lyotropic liquid crystal, by definition, consists essentially of an amphiphilic compound and a solvent. The latter modifies the structure of the amphiphile from solid or liquid to liquid crystalline by changing the environment around the polar part of the amphiphile. Numerous examples of such lyotropic mesoaggregates with biological importance have been reported in which the solvent may be water or an organic hydroxy compound. Among these organic compounds found to give nonaqueous liquid crystals were different chain length alkanediols, low molecular weight polyethylene glycols, different cellosolves and some amines. (see "Nonaqueous Lyotropic Liquid Crystals from Lecithin and Nonionic Surfactants" by Li Ganzuo et al. in Mol. Cryst. Liq. Cryst. Vol 72 (Letters), pp 183-188).
More recently, two-headed, single-chain amphiphiles have been disclosed which produce huge aggregates in dilute aqueous solutions. A flexible decaethylene unit and a rigid diphenylazomethine or biphenyl unit were found to produce aggregates with a rod-like structure so as to form a monolayer or bilayer membrane. (see J. Am. Chem. Soc. 101, 5231, 1979).
This intrinsic property of lyotropic mesomorphism exhibited by block copolymers with amphiphilic properties, results in the formation of a thermodynamically stable liquid crystalline system through the penetration of a solvent between the molecules of a crystal lattice. These lyotropic mesoaggregates usually form ordered lattices in one or two directions which cause characteristic anisotropy. See Lyotropic Liquid Crystals and the Structure of Biomembranes, edited by Stig Friberg, in a chapter titled "Lyotropic Mesomorphism--Phase Equilibria and Relation to Micellar Systems", by Ingvar Danielsson, Advances in Chemistry Series 152, published by A. C. S. 1976. This property has been used to synthesize stable, model membranes which can be used to study biological processes, particularly those characterized by a lipid bilayer membrane.
Though graft copolymers of this invention are not structurally similar to the AB type block copolymers which form liquid crystals, it was expected that the propensity of the block copolymers to form a layered structure in which the molecules are so tightly packed as to form an anisotropic mesoaggregate or liquid crystalline structure, would also likely result from the amination of the graft copolymers of this invention. If such a structure did in fact result, it was evident that further amination of the molecules, more particularly halogen-containing chains thereof, would be obstructed, except for portions of those molecules near the surface of the huge aggregates expected to be formed. It is not. Quite surprisingly, the graft copolymers of this invention may be aminated essentially completely, if so desired, thus forcing the conclusion that the structure of the aminated copolymers of this invention, whether on a submicroscopic or macroscopic scale, is quite distinct from the prior art aminated block copolymers.